1. Technical Field
The present application relates to operation of a downhole pumping system, and in particular to measurement of polished rod position of a reciprocating rod pump.
2. Description of the Related Art
In a reciprocating rod pumping system, the polished rod is connected to a sucker rod that drives a plunger of a downhole pump. Pumping fluids from downhole to the surface is achieved by the polished rod reciprocating motion induced by a prime mover using a pumpjack and a set of speed reducers, including sheaves, jackshafts and a gearbox.
The position of a polished rod obtained for the entire pumping cycle is often used for the diagnostic analysis and control of the reciprocating rod pumps. Diagnostic analysis allows for an evaluation of the performance of a reciprocating rod pump based on simultaneous measurements of the polished rod load and position as a function of time, taken at several points during a single pumping cycle. A set of such measurements is called a “surface card”, “surface dynamometer card” or simply a “DYNO card”, and allows for determination of stress distribution along the entire rod system for the entire stroke. It is also used for calculation of the “downhole card”, which represents the loads and displacements of the downhole pump plunger over the entire cycle. The downhole card is used for determination of pump production and pump fillage as well as identification of pump-off conditions, gas interference, valve leakage, insufficient tubing anchoring and a number of other conditions affecting pumping efficiency. The calculation of the downhole card can be performed in real time or any time later after collecting the data. The real time measurements of the polished rod load and position are used by most controllers that control the operation of pumping units installed on hydrocarbon producing wells, such as pump-off-controllers (POC) or variable speed controllers equipped with a variable frequency drive (VFD).
Typically, the polished rod load recorded in the surface card is obtained using a load cell or strain gauges mounted between the carrier bar and the polished rod clamps. Alternatively, it can be measured based on changes in the polished rod strain, or in case of beam pumping units, it can also be measured using beam mounted strain gauges that measure walking beam deformation. Polished rod load may also be estimated from measurements of the prime mover torque and the crank or polished rod position.
The polished rod position may be measured using a variety of sensors mounted on different moving parts of the pumping system. In one method, accelerometers are installed on the polished rod and the rate of change of speed of the polished rod is measured. However, the accuracy of this method is poor, particularly in determination of the stroke start position (especially in case of high dynamics of the polished rod movement, including vibrations).
In the case of a beam pumping unit with a walking beam, the polished rod position can be calculated from the inclination of the walking beam. The inclination can be measured using an inclinometer or an encoder sensing beam rotation (angular position). However, the accuracy of the inclinometer is adversely affected by the pumping system operating conditions, particularly in the presence of vibrations; the encoder may be prone to failure due to difficulties in mounting the parts of the encoder around the central bearing of the walking beam, which are constantly rotating with respect to each other when the pump is in operation.
Another method of determining polished rod position of a beam pumping unit consists of calculating it from the crank position. The crank position is typically inferred from motor speed and the system's reduction ratio, as well as from the initial crank angle at the start of the stroke. Typically, the start of the stroke is triggered by a sensor detecting the passage of the crank at a certain crank angle that needs to be physically measured by a technician during the calibration. Motor speed, in turn, is measured from an encoder or from a VFD. The encoder may comprise a sensor detecting motor shaft revolutions. However, the use of crank position to compute polished rod position may thus require multiple sensors or encoders, as both the motor speed and the initial crank angle must be determined, and these determinations may include inherent inaccuracies, particularly when it comes to determination of the initial crank angle. Further, installing sensors for detecting the crank start position is potentially unsafe, since the worker installing and adjusting the location of the sensor may be pinched or hit by the rotating crank or counterweight.
In a long-stroke pumping unit where the polished rod position is controlled by the movement of a chain propelled by a sprocket mounted on the gearbox output shaft, the polished rod position can be calculated based on the chain position. The chain position is determined from measurements of sprocket revolutions occurring since the beginning of the stroke that is typically triggered by detecting the passage of a selected point along the chain. The location of this point needs to be physically measured, e.g. in terms of the distance traveled by the chain until this point reaches the lowest point. The sprocket revolutions may be measured directly or based on motor revolutions, which require using an encoder when motor is not controlled by VFD.
The foregoing methods, generally, require initial calibration that may be difficult, and/or careful positioning of sensors, which in some circumstances may be inherently dangerous. Further, methods for indirectly determining polished rod position are subject to inaccuracies in the calculations used to infer the polished rod position. Any such inaccuracies may adversely affect the calculation of the surface card, particularly when the polished rod load is not measured directly, but is estimated based on the measured torque and the determined above polished rod position. Other methods require multiple sensors or sensors that might be prone to failure.
Therefore, there is a need for a system for measuring polished rod position during the entire stroke with improved accuracy and that may optionally be self-calibrating.